Generally, in the television art, broadcast television signals include audio signals referred to as a composite audio signal following the standard known as the BTSC standard for audio signals. The audio signal is a composite signal made up of several separable channel signals that are combined into a composite signal and transmitted alongside the broadcast television video signals. The BTSC audio MTS signal was developed to support stereo audio signals for broadcast television reception and includes several individual channels. The BTSC MTS signal is transmitted at a designated carrier frequency as part of the composite broadcast television video signal, as illustrated in FIG. 1, which depicts relative signal energy plotted against the carrier modulation frequency f.
FIG. 2 illustrates the composite audio signal that includes several separate channels. In FIG. 2, the aural carrier deviation is plotted against the BTSC sub-carrier frequencies f. To support monaural or “mono” sound televisions, the first signal is the main channel “L+R”, which is comparable to and useful for monaural audio signals. A television that does not support stereo sound can receive a monaural audio sound signal in this channel. For programs broadcast without stereo, this channel contains all of the audio. A pilot signal is then provided at a specified frequency fH that is typically identical with the horizontal line repetition rate of the accompanying video signal. A stereo sub-channel of “L−R” is then provided centered at frequency 2 fH. As can be understood from simple algebraic manipulations, the reception of the L+R and L−R channels provide a monaural channel and a straightforward means to recover the L and R channels separately for stereo audio reproduction, because adding the two channels results in an output of 2L, and subtracting the two channels results in an output of 2R. A channel designated the SAP channel is provided centered at a sub-carrier frequency 5 fH. The SAP channel is used to provide a Supplemental Audio Program (hence, the label “SAP”) such as a second language, for example Spanish or Chinese, for a broadcast. Finally, the standard supports a so-called “professional channel” for transmitting information useful to television professionals, but typically not processed by a home television set, at a sub-carrier frequency of 6.5 fH. The incoming analog signal is usually referred to as the Sound Intermediate Frequency signal (“SIF”), which is the audio part of the television broadcast signal the receiver circuitry operates on, as shown in FIG. 1.
FIG. 3 depicts a first prior art approach to receiving and producing the left and right stereo signals from the MTS broadcast signal. In the MTS receivers of the prior art, pure analog circuits are typically used to receive, separate, demodulate and process the channels which make up the composite audio signals for the BTSC broadcast television standard signal. As illustrated in FIG. 3, the SIF signal is coupled to an analog FM demodulator circuit 101, which demodulates the signal and removes the FM carrier and outputs the composite audio signal of FIG. 2. Typically, in a BTSC system the SIF carrier frequency amounts to 4.5 MHz, although other frequencies could be used, for example for other standards. The composite audio signal is then coupled to an analog signal processing circuit 103 that separates the various audio channels of FIG. 2 from the composite signal. Circuit 103 then outputs the corresponding audio signals L and R for reproduction by the television speakers.
One disadvantage of the prior art approach is that analog signal processing uses various discrete components which are large in utilized circuit board area, may exhibit large variations with temperature or process variances, are noise sensitive, and are not compatible with highly integrated digital circuits that can provide advanced filtering and processing capabilities in very small integrated circuit devices. Complex analog components such as filters, integrated inductors, capacitors, resistors, and the like, may be required and these are known to be difficult to build accurately in an integrated form due to process variations, temperature dependent value variations, and the like, as is known by those skilled in the art. An analog receiver for FM demodulation for television audio signals using prior art analog circuitry is shown, for example, in U.S. Pat. No. 4,490,680, to Goto, issued Dec. 25, 1984, which is incorporated herein by reference.
FIG. 4 depicts another prior art approach that uses an analog FM demodulator, or front-end receiver, coupled to an analog to digital conversion circuit and then followed by a digital signal processing circuit. In FIG. 4, system 200 implements this approach. The SIF input signal is coupled to an analog FM demodulator circuit 201 which outputs an analog composite audio signal. Analog to digital converter 203 then quantizes this signal into discrete samples using conventional analog to digital converter circuitry. The digital output signal is then processed by a digital audio processor function 205, which may be implemented as a DSP integrated circuit that may be a programmable digital signal processor, or a hardware implemented digital signal processor. One example of this prior art approach is described and illustrated, for example, in U.S. Patent Application No. 2003/0161477A1, to Wu et al, published Aug. 28, 2003, which is herein incorporated by reference. Although the use of the digital signal processor in the approach described in Wu et al. offers some added performance over the purely analog receivers of the prior art, substantial analog circuitry is still required to receive and demodulate the SIF audio signal prior to the processing by the digital audio circuitry described by Wu et al.
Recent advances in the filtering and processing algorithms used with digital signal processors make it highly desirable to perform signal processing completely in digital circuitry, that is, eliminating the analog signal processing circuits of the prior art. Further, the continuing advances in integrated circuit technology, and the availability of highly advanced programmable digital signal processors as known in the art, make signal processing in the digital domain very desirable in terms of cost, speed, and performance as well as circuit area and increased signal to noise ratio performance, eliminating the need for adjustments to compensate for component tolerances, etc.
Thus, a need exists for a purely digital signal processing method and apparatus to receive, demodulate, process and reproduce the composite audio signal for a BTSC standard broadcast television signal. The preferred embodiments and methods of the invention described herein address this need.